A laser vibration-measuring system has the advantage of being non-contact in measurement of vibration. Some applications require not only point vibration measurement but also surface vibration at several critical points by deflecting the beam at different angles by what is called the scanning technique. The scanning vibration measurement is highly essential for accurately measuring the dynamic parameters of micro devices, disc surface, etc. Since conventional mechanical movement of the object under investigation from one point to the next will not eventually result in precise measurement, which is not practicable for most applications. Present techniques of scanning in laser vibrometers apply a rotating polygonal mirror or a deflecting mirror for scanning purposes. Since the mirror is driven mechanically, the system is subjected to inaccuracy. Moreover, the scanning mirror is subjected to vibration which degrades the accuracy of vibration measurement to a considerable extent. This vibration effect is more predominant when the scanning rate is high (sweep time of scanning is less). This vibration will also result in reduction of the resolution of scanning, i.e., number of resolvable spots. Moreover, the laser beam spot size needs to be considerably reduced for microstructure inspection.
For micro devices such as components in an optical head, hard disk drive (run out, acceleration, head gimbel resonance, dynamic response of slider bearing, etc.), dynamic characteristics are important to be measured and analyzed. Point vibration measurement does not reveal complete and accurate dynamic information along the surface. So these miniaturized devices need to be moved by conventional means from point to point to investigate the device at several points, which leads to inaccuracy. Moreover, this is not possible for most of the micro devices, which require in process investigation.
Any point on the surface of a rotating target is composed of both tilt and in-plane motion and hence the resultant speckle velocity has a component of each motion type. So using a laser scanning vibrometer on rotating a target is more complicated, since the speckle pattern sampled by the detector will change spatially. This effect will produce a phase modulation of the detector output and is indistinguishable from the Doppler frequency shift associated with surface movement (dynamic characteristics) to be measured. Since the noise is linked to the vibrating frequency of interest, uncertainty is induced in the measurement, which necessitates the use of engineering judgement in the interpretation of the result. Moreover, when this speckle noise due to periodic target motion has a fundamental frequency as the vibration to be measured, then it is difficult to distinguish the noise. One method of eliminating the effect of this noise is disclosed by Pickering et al. in the paper entitled "The Laser Vibrometer: A Portable Instrument," Journal of Sound and Vibration, utilizes a rotating scattered disk as frequency shifting device, thereby the noise spectrum of the device will be comparable with the noise spectrum produced by the rotating target, and it can be canceled. Adopting the above technique will not eventually eliminate the noise effect since the rotating scattered disk does not rotate at exactly the same speed as the rotating target. Moreover, the surface of the rotating target and the rotating scatted disk may not be the same. This process will not provide an accurate remedy to the problem. Rothberg et al. in the paper entitled "Laser Vibrometry: Pseudo Vibration," Journal of Sound and Vibration, 1990, describes an engineering judgement technique to eliminate the pseudo-vibration of the speckle noise. A value above a prescribed height of the speckle noise peak, obtained by experiments is accepted as valid vibration data. This method is based on an assumption and can lead to uncertainty.
With the increase in the data storage capacity in a small area, the track density becomes higher and the distance of slider head from the disk surface becomes lower. So the measurement of fly-height in the range of nanometers is highly essential. Although there are several fly-height testers applying a white light interferometer fly-height tester, multiple wavelength fly-height tester described by C. Lacey in U.S. Pat. No. 5,280,340, monochromatic fly-height tester described in the article "Accurate Measurement of Gas-Lubricated Slider Bearing Separation using Interferometer" by T. Ohkubo and J. Kishegami, polarization phase modulated interferometric fly-height tester described by G. Sommargren in U.S. Pat. Nos. 4,606,638 and 5,218,424, phase shifting interferometric fly-height tester, etc., they are subjected to limitations. Since glass plate, which replaces the actual disk, rotating at high speed is subjected to internal stresses and hence the index of refraction varies throughout the material. This will result in the reduction in the accuracy of the system due to the change in the wavelength of the laser beam applied for measurement. Since the slider is flying obliquely, fly-height at a point will not reveal the least fly-height in the system but the fly-height at the point. So it is necessary to obtain the fly-height information at various points by scanning the beam accurately rather than using mechanical movement of the disk assembly, which is not possible using the current available systems. Also, the present systems are limited by the problem of misalignment of the measuring beam since the beam is incident at an angle rather than perpendicular to the disk surface and the slider head surface. The fly-height thus measured will be influenced by the error due to misalignment.